Novel iron-containing aluminosilicate zeolites and methods of making and using same

ABSTRACT

There is disclosed iron-containing aluminosilicate zeolites having both framework iron and iron cations on the ion-exchange sites. There is also disclosed a direct synthesis method of making an iron-containing aluminosilicate zeolite, which does not require the use of an intermediate step, such as ion-exchange or impregnation. In addition, there is disclosed a method of using the iron-containing aluminosilicate zeolite disclosed herein in a selective catalytic reduction reaction, typically in the presence of ammonia, to reduce or remove nitric oxides from exhaust emissions.

This application claims the benefit of domestic priority to U.S.Provisional Patent Application No. 60/935,436, filed Aug. 13, 2007,which is herein incorporated by reference in its entirety.

The present invention relates to a method of directly synthesizingiron-containing aluminosilicate zeolites. The present disclosure alsorelates to iron-containing aluminosilicate zeolites, wherein thestructures of the zeolite has both framework iron and iron cations onthe ion-exchange sites, as well as methods of using such zeolites,including for selective catalytic reduction (SCR) of nitrogen oxides(NOx) in exhaust gases.

Nitric oxides (NO_(x)) have long been known to be polluting gases,principally by reason of their corrosive action. In fact, they are theprimary reason for the cause of acid rain. A major contributor ofpollution by NO_(x)'s is their emission in the exhaust gases of dieselautomobiles and stationary sources such as coal-fired power plants andturbines. To avoid these harmful emissions, SCR is employed and involvesthe use of zeolitic catalysts in converting NO_(x) to nitrogen andwater.

Until the present invention, aluminosilicate zeolites for suchapplications were made by doping with a metal by ion-exchange orimpregnation. Drawbacks associated with traditional metal doping of azeolite include the necessity of a separate, additional step in themanufacture of a zeolite, and the difficulty in controlling the metalloading within the zeolite using ion-exchange or impregnation.

For example, U.S. Pat. No. 4,961,917 discloses the use of metal promoted(Fe and Cu) zeolites for reduction of NOx with ammonia. The zeolitedisclosed therein has a silica to alumina ration (SAR) of at least about10, a pore diameter of at least 7 Angstroms, and is selected from thegroup consisting of USY, Beta, and ZSM-20. As described in the examplestherein, Beta zeolite was synthesized, calcined, ion-exchanged withNH₄NO₃, and then ion-exchanged with Fe or Cu.

U.S. Pat. No. 5,451,387 discloses the use of Fe-containing ZSM-5zeolites for NOx reduction with ammonia, where iron is introduced intoZSM-5 zeolites by ion-exchange with ferrous salts. The zeolite disclosedtherein has a SAR of at least 5 and a pore diameter of less than about 7Angstroms.

U.S. Pat. Nos. 6,689,709 and 7,118,722 disclose stabilized Fe-promotedzeolites for NOx reduction. The zeolites are selected from USY, Beta,and ZSM-20 types and have a pore diameter of at least about 7 Angstroms.The Beta zeolites described therein are first steamed at hightemperature and then ion-exchanged with iron sulphate solution.

U.S. Pat. No. 6,890,501 discloses a process for simultaneously removingNOx and N₂O with ammonia using Beta zeolite loaded with iron and havinga SAR ranging between 8 and 100. The Beta zeolite disclosed therein wasprepared by ion-exchange or impregnation with Fe(NO₃)₃. Additionally, itteaches that the iron incorporated into the zeolite network duringsynthesis does not provide any catalytic activity.

The characterization of Fe-ZSM-5 zeolite, which has iron substituted inthe zeolite framework rather than aluminum, is reported in Journal ofPhysical Chemistry, vol. 89, pp. 1569-71 (1985). The preparation offerrisilicate molecular sieves, which are described as analogs to theZSM-5 zeolites, is reported in Journal of Catalysis, vol. 100, no. 2,pp. 555-7 (1986).

U.S. Pat. No. 4,952,385 discloses a process for preparing analuminum-free ferrisilicate ZSM-5 zeolite and that such a zeolite isuseful in Fischer-Tropsch synthesis to directly convert mixtures ofcarbon monoxide and hydrogen to hydrocarbons. Finally, preparation andcharacterization of substantially aluminum-free ferrisilicate mordeniteis reported in Zeolites, vol. 11, no. 1, pp. 42-7 (1991).

Despite the abundance of prior art in this area, the art is silent onmethods of making zeolites containing transition metals that do notrequire some intermediate step, such as ion-exchange or impregnation, oran additional step, such as steam treatment, to extract the transitionmetal from the zeolite framework when made by direct synthesis. Thus,there is a need for an improved and simplified method of makingmetal-containing aluminosilicates that does not require ion-exchange orimpregnation or extraction of the cations from the zeolite frameworkafter direct synthesis and the products thereof. To that end, theInventors have discovered iron-containing aluminosilicate zeoliteswherein the structure of the zeolite has both framework iron and ironcations on the ion-exchange sites, and methods of directly synthesizingthese iron-containing aluminosilicate zeolites.

SUMMARY

There is disclosed an iron-containing aluminosilicate zeolite whereinthe structure of the zeolite has both framework iron and iron cations onthe ion-exchange sites.

There is further disclosed a method of making an iron-containingaluminosilicate zeolite, comprising: mixing a source of alumina, asource of silica, optionally an organic structural directing agent, andwith an iron constituent to form an iron containing synthesis mixture;and performing at least one heating step on the iron containing mixtureto form an aluminosilicate zeolite wherein the structure of the zeolitehas both framework iron and iron cations on the ion-exchange sites.

As the inventive method does not require calcining prior to metaladdition, it is possible to coat a monolith with an uncalcinediron-containing aluminosilicate zeolite described herein, and calcinethe entire coated structure. Thus, there is also disclosed a monolithcoated with an iron-containing aluminosilicate zeolite wherein thestructure of the zeolite has both framework iron and iron cations on theion-exchange sites.

In addition, there is disclosed a method of using the iron-containingaluminosilicate zeolite disclosed herein in an SCR reaction, typicallyin the presence of ammonia or source thereof, such as a urea solution.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a graph of catalytic behavior of various inventive andcomparable fresh Fe-containing ZSM-5 materials in the selectivecatalytic reduction of NO_(x).

FIG. 2. is a graph of catalytic behavior of various inventive andcomparable hydrothermally aged Fe-containing ZSM-5 in the selectivecatalytic reduction of NO_(x).

FIG. 3. is a graph of catalytic behavior of various inventive andcomparable hydrothermally aged Fe-beta materials in the selectivecatalytic reduction of NO_(R).

FIG. 4. is a graph of catalytic behavior of hydrothermally agedFe-chabazite in the selective catalytic reduction of NO_(x).

FIG. 5. is a graph of Hydrogen Temperature-programmed reduction (H₂-TPR)profiles for fresh samples of Example 1, 2 and 4.

Aside from the subject matter discussed above, the present disclosureincludes a number of other exemplary features such as those explainedhereinafter. It is to be understood that both the foregoing descriptionand the following description are exemplary only.

DETAILED DESCRIPTION OF THE INVENTION

Zeolites are the aluminosilicate members of the family of microporoussolids known as “molecular sieves”. This phrase refers to the ability toselectively sort molecules based primarily on a size exclusion process,and is a function of the regular pore structure of molecular dimensionsassociated with zeolites. The maximum size of the molecular or ionicspecies that can enter the pores of a zeolite is controlled and definedby the diameters of the rings. For example, the term “10 ring” or morecommonly “10R”, refers to a closed loop that is built from 10tetrahedrally coordinated silicon (or aluminium) atoms and 10 oxygenatoms. The pore openings for all rings of one size are generally notidentical, due to various effects, including bond-induced strain. Thepresent disclosure is directed to a variety of zeolites, including thoseencompassed by 8R to 18R zeolites, and methods of making those zeolites.

As stated, SCR is a valuable tool in the reduction of nitric oxidesfound in polluting gases. NH₃ storage is a central feature of NH₃-SCR(selective catalytic reduction of nitrogen oxides using NH₃) catalysts,because NO_(x) (NO+NO₂) in exhaust gas can react with stored NH₃ in theabsence of NH₃ in the feed. NH₃ storage sites help reduce ammonia slipand can also allow flexibility for the NH₃ dosing strategy. FrameworkAl-sites in zeolites are acidic and adsorb NH₃ readily and are thussuitable as NH₃ storage sites. The importance of acid sites for NH₃-SCRhas been discussed in Applied Catalysis B: Environmental, vol. 66, pp.208-216, 2006.

NO oxidation is an important step in the overall NH₃-SCR reaction,especially in cases where NO₂ is absent in the feed, i.e. when theNO/NO_(x) ratio is close to unity. Fe-exchanged zeolites have beenstudied for NH₃-SCR, and NO oxidation has been suggested to be a keystep for the reaction on these materials (Applied Catalysis A: General,vol. 235, pp. 241-251, 2002).

The iron-containing aluminosilicate zeolites described herein containacid sites, framework iron, and exchangeable Fe-sites. Aluminum atoms inthe zeolite framework create acid sites that are necessary for NH₃storage for SCR reaction. Part of Fe is in the framework after synthesisand part of Fe will be present at exchange sites when the Fe content inthe material is sufficiently high. Such iron-containing zeolite as freshis active for NH₃-SCR reaction. The framework iron will migrate out ofthe framework during hydrothermal aging, and the framework Fe thenbecomes a source for providing additional Fe to exchange sites neededfor SCR reaction. The novel iron-containing zeolites are more durablefor NH₃-SCR reaction upon hydrothermal aging or in a practical use thana conventional Fe-exchanged zeolite or Fe-silicate without aluminum.

In one embodiment, there is disclosed an iron-containing aluminosilicatezeolite, wherein the structure of the zeolite has both framework ironand iron cations on the ion-exchange sites. The phrase “framework iron”(or any version thereof) refers to iron cations present in the frameworkof an aluminosilicate zeolite to balance the negative charge of thezeolite. Framework iron is inactive and generally the result of directsynthesis, not ion-exchange or impregnation. The phrase “iron cations onion-exhange sites” (or any version thereof) refers to iron cations thatare not in the zeolite framework and are active for NO_(x) reduction.Iron cations on ion-exhange sites may result from ion-exchange or, as inthe present case, may also result from direct synthesis.

The inventors have discovered the product according to the presentdisclosure wherein the structure of the zeolite has both framework ironand iron cations on the ion-exchange sites. Therefore, the zeolites ofthe present invention are active even in the absence of an activationstep and they remain active after exposure to steam and/or heat.

In another embodiment, there is disclosed a method of makingiron-containing aluminosilicate zeolites that is based on the ability toadd an iron constituent during the synthesis step of the zeolite. Thephrase “direct synthesis” (or any version thereof) refers to a methodthat does not require a metal-doping process after the zeolite has beenformed, such as a subsequent ion-exchange or impregnation method. TheInventors have discovered that the direct synthesis method according tothe present disclosure makes it easier to control the amount of iron inthe zeolite than traditional ion-exchange and impregnation methods.Thus, not only is the disclosed method simpler than traditional methods,but it produces a better final product since the iron content can bemore accurately controlled. Moreover, in at least one aspect of thepresent invention, the method does not require an additional step toextract the transition metal from the zeolite framework, such as steamtreatment, as active iron is already present in the zeolite.

In addition, the iron-containing aluminosilicate zeolites made accordingto the present disclosure are hydrothermally stable. As used herein“hydrothermally stable” refers to the ability to retain at least somecatalytic activity under high temperature conditions of use, such asunder traditional driving conditions of an automobile. In oneembodiment, zeolites made according to the present disclosure arehydrothermally stable at 700° C. for 16 hours.

In another embodiment, the resulting iron-containing aluminosilicatezeolite is hydrothermally stable in the presence of sulfur compoundssince sulfur or sulfur compounds are typically found in exhaust gases ofcoal-fired power plants and of turbines or other engines fueled withsulfur-containing fuels, such as fuel oils and the like.

As stated, there is disclosed a method of making a iron-containingaluminosilicate zeolite that comprises adding an iron constituent duringsynthesis of the zeolite. In one embodiment, the direct addition of theiron constituent enables the synthesis of an aluminosilicate zeolitewith at least 0.2% by weight, and more typically from 1.0 to 5.0% byweight of iron, such as Fe2+ and Fe3+.

Prior to the present invention, adding these amounts of iron to zeoliteswere primarily done via ion-exchange or impregnation, after the zeolitewas calcined to form a final product. As stated, the inventive methodadds the iron constituent during the synthesis process, and not on thefinished zeolite, e.g., not after calcination. The present inventionalso does not require an additional step to extract the transition metalfrom the zeolite framework, such as steam treatment. Thus, anotherbenefit of the present invention is that it enables the inventivematerial to be coated on a monolithic substrate and the entire substratebe calcined.

As stated, the iron-containing aluminosilicate zeolite described hereinis made via a direct synthesis process. However, it is possible toinclude additional iron cations and/or at least one additional metalcation on the ion-exchange sites via an ion-exchange process. In oneembodiment, the at least one additional metal is chosen from copper,manganese, cobalt, and silver.

The iron-containing aluminosilicate zeolite described herein may exhibita selective catalytic reduction of nitrogen oxides with NH₃ or urea ofgreater than 40% conversion at temperatures at 250-300° C. in exhaustgases prior to aging or exposure to steam.

In one embodiment, the iron-containing aluminosilicate zeolite describedherein exhibits a selective catalytic reduction of nitrogen oxides withNH₃ or urea of greater than 80% conversion at temperatures between 300°C. and 500° C. in exhaust gases after hydrothermal aging or exposure tosteam.

There is also disclosed a direct synthesis method of making aniron-containing aluminosilicate zeolite that comprises:

mixing a source of alumina, a source of silica, optionally a structuraldirecting agent, with an iron constituent to form an iron containingsynthesis mixture; and

performing at least one heating step on the iron containing synthesismixture to form an iron-containing aluminosilicate zeolite wherein thestructure of the zeolite has both framework iron and iron cations on theion-exchange sites.

Generally, the present method is directed to making by direct synthesisany type of iron-containing aluminosilicate zeolite by mixing a sourceof alumina, a source of silica, a source of iron, and optionally with astructural directing agent to directly synthesize iron-containingaluminosilicate zeolites. Depending on the desired zeolite product, suchas a Beta, ZSM-5 or Y-type zeolite, the use of structural directingagent could vary. For example, in one embodiment, there is disclosed amethod of making iron-Beta zeolites by mixing the sources of alumina,silica and iron with water and an organic structural directing agent,such as tetraethylammonium hydroxide (TEAOH).

In a different embodiment, there is disclosed a method of making a ZSM-5zeolite without the use of an organic structural directing agent.

Non-limiting examples of sources of alumina that may be used in thepresent disclosure include sodium aluminate, aluminum hydroxide,alumina, aluminum nitrate, and aluminum sulfate. The method may comprisean additional step of removing any residual sodium from the product.This is typically done via an ion-exchange process with known salts orthe like, including ammonium salts of Cl, SO₄, NO₃. In one embodiment,residual sodium is removed prior to calcining by slurrying the productin a desired salt, such as NH₄NO₃.

The source of iron is typically an iron salt is chosen from ferricnitrate, ferric chloride, ferrous chloride, and ferrous sulfate. . Inaddition, the source of silica may comprise a silica sol, which istypically added under vigorous stirring conditions. Non-limitingexamples of other sources of silica that might be used include knownsilicates, such as sodium silicate, and sodium metalsilicate, as well ascolloidal silica, silica gel, precipitated silica, silica-alumina, andthe like.

After formation of the gel, a zeolite source, such as a beta-zeolite andZSM-5, is optionally added in the form of crystallization seeds. Whenused, the crystallization seeds are added in amounts ranging from 0.1 to5.0% by weight.

Next, the gel is heated to form a product. Heating of the gel may beperformed in an autoclave at a temperature ranging from 140° C. to 250°C. for a time ranging from 4-72 hours, such as 24-48 hours. Thetreatment temperature may be lowered when a zeolite-Y is synthesized.For example, the temperature may range from 80 to 105° C., if a Y-typezeolite is formed.

After cooling and optionally performing at least one treatment processon the product chosen from filtering, washing and drying, it iseventually calcined to form an iron-containing aluminosilicate zeolitecontaining at least 0.2%, such as from 1.0% to 5.0% by weight of Fe2+ orFe3+. As explained in more detail below, the calcination step may beperformed after the inventive iron-containing aluminosilicate zeolite iscoated on a monolith.

In one embodiment, the product is further subject to an ion-exchangeand/or impregnation step to increase the amount of iron or add at leastone additional metal, such as copper, manganese, cobalt, and silver.

In another embodiment, there is disclosed a hydrothermally stableiron-containing aluminosilicate zeolite made according to previouslydescribed method, including one containing at least 0.2% by weight ofFe2+ or Fe3+ made by a direct synthesis method.

The iron-containing aluminosilicate zeolite described herein typicallyhas a silica to alumina ratio of 5 to 100, or less than 60, or even5-50, depending on the type of zeolite. For example, a Y-type zeolitetypically has a silica to alumina ratio of 5 to 40, a beta-zeolite has asilica to alumina ratio of 10-80, and a ZSM-5 zeolite has a silica toalumina ratio of 15 to 100.

In addition to the inventive method of making and the inventiveiron-containing aluminosilicate zeolite, there is disclosed a method ofusing the disclosed iron-containing aluminosilicate zeolite. Forexample, a typical exhaust gas of a diesel engine contains from about 2to 15 volume percent oxygen and from about 20 to 500 volume parts permillion nitrogen oxides (normally comprising a mixture of NO and NO₂).The reduction of nitrogen oxides with ammonia to form nitrogen and H₂Ocan be catalyzed by metal-promoted zeolites, hence the process is oftenreferred to as the “selective” catalytic reduction (“SCR”) of nitrogenoxides.

Thus, in one embodiment there is also disclosed a method of SCR ofnitrogen oxides in exhaust gases which comprises at least partiallycontacting an exhaust gas with the iron-containing aluminosilicatezeolite disclosed herein.

In order to reduce the emissions of nitrogen oxides various exhaustgases, ammonia is typically added to the gaseous stream containing thenitrogen oxides. In one embodiment of the present invention, ammonia isused to allow the gaseous stream, when contacted with the inventiveferrialuminosilicate zeolite at elevated temperatures, to catalyze thereduction of nitrogen oxides.

In one embodiment, a urea solution may be used to provide the ammonia tothe gaseous stream. This is particularly true when used in automotiveexhaust treatment applications and stationary NO_(x) reductionapplications.

Non-limiting examples of the types of exhaust gases that may be treatedwith the disclosed zeolites include both automotive exhaust, from on andoff road vehicles, including diesel engines. In addition, exhaust fromstationary sources, such as power plants, stationary diesel engines, andcoal-fired plants, may be treated. Thus, there are also disclosedmethods of treating exhaust emissions, such as automotive exhaust orexhaust from stationary sources.

The iron-containing aluminosilicate zeolite of the present invention maybe provided in the form of a fine powder which is admixed with or coatedby a suitable refractory binder, such as alumina, bentonite, silica, orsilica-alumina, and formed into a slurry which is deposited upon asuitable refractory substrate. In one embodiment, the carrier substratemay be “honeycomb” structure. Such carriers are well known in the art ashaving a many fine, parallel gas flow passages extending therethrough.One non-limiting examples of the material used to make the honeycombstructure comprises cordierite, mullite, silicon carbide, alumina,titania, zirconia, silica, alumina-silica, alumina-zirconia, stainlesssteel, Fe—Cr—Al alloy and the combinations thereof.

There is also disclosed a method of making a monolith coated with anuncalcined iron-containing aluminosilicate zeolite. As the inventivemethod does not require ion-exchange to be performed on a calcinedstructure, it is possible to coat a monolith with an uncalcined zeolitedescribed herein, and calcine the entire coated structure. This would bebeneficial in that it would further ease the processing steps of thezeolite manufacturer.

This method would comprise mixing a source of alumina, a source ofsilica, and a structural directing agent with an iron constituent toform an iron containing product and performing at least one heating stepon the iron containing product to form a iron-containing aluminosilicatezeolite having at least 0.2%, such as an amount ranging from 1 to 5% byweight of Fe2+ or Fe3+. Once the washcoat or uncalcined zeolite is made,it can be directly coated onto a monolith. The entire coated monolithcould then be calcined to remove the structural directing agent.

In another embodiment, the iron-containing aluminosilicate zeolite maybe provided in discrete forms (as opposed to a coating on a substrate).Non-limiting examples of such forms include pellets, tablets orparticles of any other suitable shape, for use in a packed bed, forexample. The iron-containing aluminosilicate zeolite according to thepresent invention may also be formed into shaped pieces such as plates,tubes, or the like.

Unless otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

The invention will be further clarified by the following non-limitingexamples, which are intended to be purely exemplary of the invention.

EXAMPLES Example 1 Fe-Containing ZSM-5 (2.2% Fe, SAR=31) byDirect-Synthesis Without Organic Structure Direction Agent

550 grams of sodium silicate solution (28% SiO₂, 9% Na2O), 24.6 grams ofFe(NO₃)3·9H₂O, and 540 grams of de-ionized water (DI-H₂O) were mixedtogether. A solution consisting of 46 grams of Al₂(SO₄)₃.18H₂O, 21 gramsof H₂SO₄ (96%) and 250 grams of DI-H₂O was slowly added to the mixturewith vigorous stirring. Finally 3.0 grams of ZSM-5 zeolite was added tothe synthesis gel as crystallization seeds. The gel had the followingmolar composition.

33 SiO₂:1.0 Al₂O₃:0.4 Fe₂O₃:10 Na₂O:825 H₂O

The gel was loaded into a 2-liter autoclave and heated to 185° C. for 24hours. After cooling, the product was recovered by filtration andwashing. X-ray diffraction pattern of the product showed pure-phaseZSM-5 zeolite.

To remove residual sodium, the solid was slurried in a 3 M NH₄NO₃solution and stirred at 80° C. for 2 hours. After filtering, washing,and drying, the final product had silica-to-alumina ratio (SAR) of 31and 2.2 wt. % Fe. The BET surface area of the product was 404 m²/g andmicropore volume was 0.14 cc/g.

Example 2 (Comparable) Fe-Containing ZSM-5 (0.6 wt. % Fe, SAR=88)Prepared in Presence of Organic Structure Directing Agent

This comparable synthesis was made following the procedure described inJournal of Catalysis, vol. 195, pp. 287-297, 2000 and Example 1 of U.S.Patent Application 2006/0088469 A1.

9.6 grams of NaOH pellets (98 wt. %) was dissolved in 817 grams ofDI-H₂O, and then 61.0 grams of tetrapropylammonium hydroxide solution(TPAOH, 40 wt. %) was added. 250 grams of tetraethyl orthosilicate(TEOS) was mixed in the above solution. Finally a solution containing9.0 grams of Al(NO₃)₃.9H₂O, 2.8 grams of Fe(NO₃)₃.9H₂O, and 155 grams ofDI-H₂O was slowly added with vigorous stirring. The final mixture hadthe following molar composition.

100 SiO₂:1.0 Al₂O₃:0.3 Fe₂O₃:9.8 Na₂O:4500 H₂O

The mixture was stirred for 30 minutes and then transferred to a 2-literautoclave and heated to 175° C. without agitation. Crystalline ZSM-5 wasobtained after 2 days with identical crystal morphology shown in Journalof Catalysis, vol. 195, pp. 287-297, 2000. The solid was filtered,washed, and dried in the oven. The solid was then calcined at 550° C.for 10 hours to remove the organic agent. To remove residual sodium, thecalcined material was slurried in a 2 M NH₄NO₃ solution and stirred at80° C. for 2 hours. After filtering, washing, and drying, the finalproduct had SAR of 88 and 0.58 wt. % Fe. The BET surface area of thecalcined product was 432 m²/g and micropore volume was 0.15 cc/g.

Example 3 (Comparable) Fe-Containing ZSM-5 (3% Fe, No Aluminum) Preparedin Presence of Organic Structure Direction Agent

This comparable synthesis without aluminum was following the proceduredescribed in the Example 1 of U.S. Pat. No. 4,952,385.

550 grams of sodium silicate (28.2 wt. % SiO₂, 8.9 wt % Na₂O) from PQCorporation was mixed in 550 grams of DI-H₂O. Then a solution containing22.7 grams of Fe(NO₃)₃.9H₂O and 442 grams of DI-H₂O was added. 41.0grams of H₂SO₄ (96 wt. %) was slowly added into the mixture withvigorous stirring. Finally 139 grams of tetrapropylammonium bromidesolution (TPABr, 35 wt. %) was added. The final mixture had thefollowing molar composition.

92 SiO₂:1.0 Fe₂O₃:27.9 Na₂O:14.4 H₂SO₄:2852 H₂O

The mixture was transferred to a 2-liter autoclave and heated to 170° C.for 6 days without agitation. The solid was filtered, washed, and driedin the oven. X-ray diffraction pattern showed the solid was a pure ZSM-5phase. The solid was then calcined at 550° C. for 6 hours to remove theorganic agent. To remove residual sodium, the calcined material wasslurried in a 2 M NH₄NO₃ solution and stirred at 80° C. for 2 hours.After filtering, washing, and drying, the final product had a Si/Femolar ratio of 39 and 2.3 wt. % Fe. The BET surface area of the calcinedproduct was 427 m²/g and micropore volume was 0.14 cc/g.

Example 4 (Comparable) Fe-ZSM-5 (0.9 wt. % Fe, SAR=23) By AqueousIon-Exchange

Commercial ZSM-5 zeolite from Zeolyst (CBV 2314) was ion-exchanged withan FeSO₄ solution at 80° C. for 2 hours. After filtering, washing anddrying, the Fe-ZSM-5 product had 0.9 wt. % Fe, and BET surface area of434 m²/g.

Example 5 Fe-Containing Zeolite Beta (1.1 wt. % Fe, SAR=24) ByDirect-Synthesis

340 grams of tetraethylammonium hydroxide solution (35% TEAOH), 114grams of sodium aluminate solution (23.5% Al₂O₃ and 19.6% Na₂O), and 45grams of de-ionized water were mixed together. To this solution, 32grams of Fe(NO₃)₃.9H₂O were added. 1000 grams of sodium stabilizedsilica sol (40% silica) were added to the above mixture under vigorousstirring. Finally 15 grams of Beta zeolite was added to the synthesisgel as crystallization seeds. The gel has the following molarcomposition.

25.7 SiO₂:1.0 Al₂O₃:0.15 Fe₂O₃:1.7 Na₂O:3.1 TEAOH:198 H₂O

The gel was loaded into a 2 liter autoclave and heated to 160° C. for 2days. After cooling, the product was recovered by filtration andwashing. X-ray diffraction pattern of the product showed pure-phase Betazeolite.

To remove residual sodium, the product was then slurried in a 2 M NH₄NO₃solution and stirred at 80° C. for 2 hours. After filtering, washing,and drying, the product was calcined at 550° C. for 10 hours. The finalproduct had silica-to-alumina ratio (SAR) of 24 and 1.1 wt. % Fe. TheBET surface area of the calcined product was 728 m²/g and microporevolume was 0.20 cc/g.

Example 6 Fe-Containing Zeolite Beta (1.5 wt. % Fe, SAR=25) ByDirect-Synthesis

The synthesis procedures of Example 9 were repeated except that theamount of Fe(NO₃)₃.9H₂O added was 43.5 grams. The gel composition is:

25.7 SiO₂:1.0 Al₂O₃:0.21 Fe₂O₃:1.7 Na₂O:3.1 TEAOH:198 H₂O

After contacting with NH₄NO₃ solution and calcination, the final producthad SAR of 25 and 1.5 wt. % Fe. The BET surface area of the calcinedproduct was 718 m²/g and micropore volume was 0.20 cc/g.

Example 7 (Comparable) Fe-Beta (1.0 wt. % Fe, SAR=25) By AqueousIon-Exchange

Commercial Beta zeolite from Zeolyst (CP 814E, SAR=25) was ion-exchangedwith FeCl₂ solution at 80° C. for 2 hours. After filtering, washing anddrying, the Fe-Beta product had 1.0 wt. % Fe and BET surface area of 698m²/g.

Example 8 Fe-Containing High-Silica Chabazite (1.1% Fe, SAR=34) ByDirect-Synthesis

584 grams of N,N,N-Trimethyl-1-adamantammonium hydroxide solution(TMAAOH, 13 wt.%) were mixed with 516 grams of de-ionized water. Then18.2 grams of NaOH and 11.3 grams of alumina (53 wt. % Al₂O₃) wereadded. To this mixture, 10.5 grams of Fe(NO₃)₃.9H₂O were added. 150grams of dried silica gel (˜90% silica) were added to the above mixtureunder vigorous stirring. Finally 3 grams of chabazite zeolite was addedto the synthesis gel as crystallization seeds. The gel has the followingmolar composition.

40.0 SiO₂:1.0 Al₂O₃:0.22 Fe₂O₃:3.8 Na₂O:6.0 TMAAOH:1000 H₂O

The gel was loaded into an autoclave and heated to 160° C. for 96 hours.After cooling, the product was recovered by filtration and washing.X-ray diffraction pattern of the product showed pure-phase chabazitezeolite (CHA).

The product was calcined at 550° C. for 10 hours. To remove residualsodium, the product was then slurried in a 2 M NH₄NO₃ solution andstirred at 80° C. for 2 hours. After filtering, washing, and drying, thefinal product had silica-to-alumina ratio

(SAR) of 34 and 1.1 wt. % Fe. The BET surface area of the product was817 m²/g and micropore volume was 0.30 cc/g.

NH₃-SCR of NO_(x) With Ferrialuminosilicate Zeolites

The activities of Fe-containing zeolites for NO_(x) conversion using NH₃as reductant were evaluated in a flow-through type reactor. Powderzeolite samples were pressed and sieved to 35/70 mesh and loaded into aquartz tube reactor. The gas stream contained 500 ppm NO, 500 ppm NH₃,5% O₂, and balance N₂. The hourly space velocity for all reactions was50,000 h⁻¹. Reactor temperature was ramped and NO_(x) conversion wasdetermined with an infrared analyzer at each temperature interval. Theresults are shown in FIGS. 1-4.

FIG. 1 compares SCR of NO_(x) with NH₃ on fresh Fe-containing ZSM-5materials made in Examples 1-4 above. The samples from comparableExample 2 having low Fe content and high SAR (88) and Example 3 havingno aluminum show much lower activity than the sample from inventiveExample 1 having high Fe-content and low SAR (31). Example 4, which isFe-ZSM-5 made by Fe-exchange, shows the highest activity.

FIG. 2 compares SCR of NO_(x) with NH₃ on Fe-containing ZSM-5 materialsmade by Examples 1-4 above and hydrothermally aged at 700° C. for 16hours in 10% by volume of water vapor. The sample from Example 1 showsthe highest activity in the low temperature region and also has stableactivity in the high temperature region.

FIG. 3 compares SCR of NOx conversion with NH3 on Fe-Beta materials madeby Examples 5-7 above and hydrothermally aged at 700° C. for 16 hours in10% by volume of water vapor.

FIG. 4 shows SCR of NOx conversion with NH3 on an Fe-chabazite materialmade by Example 8 above and hydrothermally aged at 700° C. for 16 hoursin 10% by volume of water vapor.

FIG. 5 shows a Hydrogen Temperature-programmed reduction (H₂-TPR)profiles for fresh samples of Example 1, 2 and 4. [Ramping rate: 10°C./min. Gas: 15 cc/min 5% H₂/Ar. The H₂ consumption was measured using athermal conductivity detector.]

Peaks in the 550-700° C. region correspond to framework iron, whereasthe 300-500° C. region correspond to exchanged Fe cations. Example 4made by an exchange process contains exchanged Fe but no framework Feand has peaks in the 300-500° C. region corresponding to reduction ofexchanged trivalent Fe to divalent Fe. Example 2 shows no evidence ofexchanged Fe, but only framework Fe with a reduction peak around 650° C.Fresh sample of Example 1 contains framework Fe (peak ˜600° C.) and ashoulder at 450-500° C. corresponding to Fe on the exchange sits. Thesamples with larger amounts of iron on exchange sites are more activefor NH₃-SCR of NO (see FIG. 1).

1-31. (canceled)
 32. A method of selective catalytic reduction ofnitrogen oxides in exhaust gases, said method comprising: at leastpartially contacting said exhaust gases with an iron-containingaluminosilicate zeolite comprising both framework iron andextra-framework iron cations.
 33. The method of claim 32, wherein saidselective catalytic reduction occurs in the presence of ammonia, urea,or sources thereof.
 34. The method of claim 32, wherein said iron ispresent in an amount ranging from 1 to 5% by weight of said zeolite. 35.The method of claim 32, wherein said zeolite has a silica-to-aluminaratio (SAR) of less than
 60. 36. The method of claim 32, wherein saidzeolite is chosen from Beta, ZSM-5, Y, mordenite, chabazite, andferrierite type zeolites.
 37. The method of claim 32, wherein saidselective catalytic reduction of nitrogen oxides improves afterhydrothermal aging said zeolite at 300° C. or higher.
 38. The method ofclaim 32, wherein said exhaust gas is automotive exhaust.
 39. The methodof claim 32, wherein said exhaust gas is from a stationary source chosenfrom a power generation plant or a stationary diesel engine.